A magnetic field disturber is here defined as being any object which alters or deforms a magnetic field emitted in proximity. For example, the disturber may be a conducting item. In this case, the alteration of the magnetic field is due to eddy current appearing in the conducting item. The conducting item is for example a metallic item. The disturber can also be a magnetic item such as a paramagnetic, ferromagnetic or diamagnetic item. In the case of ferromagnetic items, the alteration of the magnetic field is due to the fact that this item deforms the lines of the magnetic fields.
When a magnetic disturber is present in proximity to a magnetic field source and to a magnetic field transducer, the magnetic field measured by the transducer is different from that which would have been measured in the absence of this magnetic disturber. The measurement is then said to be “disturbed” by the magnetic disturber.
Here, the subsets of measurements comprising at least one measurement disturbed by a magnetic disturber are called “disturbed subset”. Conversely, the subsets of measurements not comprising any measurement disturbed by a magnetic disturber are called “healthy subset” or “undisturbed subset”.
Typically, the methods for identifying subsets disturbed by a magnetic disturber are used within methods for locating an object in a frame of reference.
Known methods for locating an object in a frame of reference comprise:
the emission of magnetic fields and the measurement of these magnetic fields with the aid of N monoaxial source/monoaxial transducer pairs so as to obtain a set of N real measurements, each real measurement of this set being obtained with the aid of a monoaxial source/monoaxial transducer pair being distinguished from the others by:                the position of the monoaxial source used to emit the magnetic field, or        the position of the monoaxial transducer used to measure the magnetic field emitted, or        the frequency of the magnetic field.        
the location of the object in the frame of reference on the basis of the measurements carried out by the monoaxial source/monoaxial transducer pairs.
A monoaxial source of magnetic field is a source which preferably emits the magnetic field along a single axis. For example, a coil whose turns are wound around one and the same axis is a monoaxial source of magnetic field and the preferential emission axis coincides with the winding axis of the turns.
In a similar manner, a monoaxial transducer is a transducer capable of measuring the projection of the magnetic field onto a single measurement axis.
Location of the object on the basis of magnetic fields exhibits numerous advantages. However, in practice, there exist numerous magnetic disturbers in the environment of the object which are liable to disturb the measurements of the magnetic field and therefore to falsify location of the object.
To correctly locate the object despite the presence of magnetic disturbers, it has already been proposed to use redundancy of the measurements. Indeed, given the short range of the magnetic disturbances, it might be expected that the presence of a magnetic disturber disturbs only a subset of the measurements. Thus, if the measurements are redundant, there often exists at least one undisturbed subset of measurements. To obtain redundancy of the measurements, the number N of monoaxial source/monoaxial transducer pairs is greater than the minimum number Nmin of measurements necessary to compute the position of the object in the frame of reference.
Thereafter, during location of the object, more weight can be given to the undisturbed measurements than to the disturbed measurements so as to obtain correct location of the object despite the presence of the magnetic disturber. For example, the position of the object is only computed on the basis of the undisturbed subset of measurements.
However, before this, it is necessary to implement a method for identifying the disturbed subset or subsets of measurements.
Hitherto, the determination of the disturbed subsets has been carried out during a prior calibration step (see for example patent application U.S. 2008/0033282). This prior calibration step comprises the construction of an estimation Pi of the position of the object in the frame of reference on the basis of an observer Oi involving only a subset Mi of the N measurements.
The estimation Pi is then compared with the known position of the object so as to determine whether or not the subset Mi comprises disturbed measurements.
This method does not operate correctly if the magnetic disturber is present only intermittently or if it moves in the frame of reference. Moreover, recourse to a prior calibration step is irksome.